This invention relates to a brake booster for boosting a brake operational force by means of utilizing gas pressure, more particularly, to an improvement in a stroke enlarging type booster.
A stroke enlarging type booster herein referred to is a brake booster wherein a power piston is disposed in a manner of bisecting a booster casing so as to be operated by a pressure difference between the pair of divided chambers. The power piston is separated from a controlling mechanism which controls the pressure difference due to an operation of an input member, so as to be shiftable in relation to the controlling mechanism. This stroke enlarging type brake booster is capable of obtaining a larger output stroke than the stroke input, unlike ordinary brake boosters wherein the output stroke is always the same as or smaller than the input stroke. This type of stroke enlarging brake booster is described in pending U.S. patent application Nos. 919,071, now U.S. Pat. No. 4,242,943, and 46,046. In the former a spring means for transmitting a part of operational force of the power piston to the controlling mechanism is used, while in the latter it is not. Both boosters are almost identical otherwise.
Ensuing studies in this field revealed that there was still room for improvement. Although there is little problem in a stroke enlarging type brake booster with a power piston which is actuated by a large difference of pressure acting on opposite sides thereof, in a case wherein boosting force, that is, the above-mentioned difference of pressure has a certain highest limit a problem arises that an attempt of obtaining an output force over a critical point, which corresponds to the highest limit of the pressure difference, invites a temporary rapid increase of pedal stroke alone without bringing about a corresponding expected increase of braking force, which is a seriously disadvantageous phenomenon.